This invention relates generally to injection molding machines, and more particularly, to injection molding machines including means for controlling flow of molten material into a mold cavity of a machine molding assembly designed for molding a centrally apertured record disc, such as a video information disc, and for molding a central aperture into such disc.
An example of an injection molding machine of this general type is disclosed in copending and commonly assigned patent applications U.S. Ser. No. 847,367, now abandoned filed in the name of J. R. Holmes et al., and entitled "METHOD AND MEANS FOR REPLICATING CENTRALLY APERTURED VIDEO DISC RECORDS", and U.S. Ser. No. 031,205, now abandoned filed in the name of J. R. Holmes, and entitled "MOLDING APPARATUS FOR PRODUCING CENTRALLY APERTURED RECORD DISCS". The machines described in these applications comprise injection molding machines having a molding assembly with first and second mold halves reciprocally movable between a closed position defining an annular mold cavity into which molten disc-forming material of plastic or the like is injected to form a video information disc, and an open position wherein the mold cavity parting line is opened and the resultant molded video information disc is removed from the machine. The mold cavity is lined by one or more disc-shaped stamping dies carried by the mold halves and these stamping dies include discontinuous surface configurations representive of the information to be formed on the disc.
In injection molding machines of this general type, it is well known to inject molten plastic material under pressure from an injector gun through a sprue bushing and into the mold cavity. When the mold cavity is filled with the molten plastic material, the respective mold halves and the sprue bushing are cooled by a circulating liquid coolant to reduce the temperature of the molten plastic material and thereby solidify the material. After solidification, the molded information disc is removed from the machine and the cycle restarted, thereby allowing a relatively rapid production rate of the information discs. However, the plastic material within the sprue bushing typically has a relatively large thickness compared with the plastic material within the mold cavity, whereby the plastic material within the sprue bushing takes the longest time to solidify. The cycle time for molding the information discs in machines of this type therefore tends to be dependent upon the longer cooling time of the material within the sprue bushing, and not upon the shorter cooling time of the material within the mold cavity. Moreover, this material within the sprue bushing when solidified comprises wasted plastic material which must be removed from the molded information disc as by means of a punch assembly or the like, to yield the desired thin, substantially planar information disc including the required central aperture.
Formation of the central aperture by means of a conventional punch assembly as disclosed generally in the above-referenced copending patent applications poses a variety of problems in the production of the information discs. For example, the punch step requires a certain cycle time to perform, thereby prolonging the cycle time for each information disc. Moreover, the use of the punch assembly to form the central aperture results in an aperture shape having a concentricity dependent upon the state of wear of the punch die. As punch die wear progresses, the likelihood of the creation of debris in the vicinity of the aperture increases, and such debris can deleteriously affect the quality of subsequently molded information discs. Finally, the use of a punch assembly places the information disc under substantial stress which occasionally can result in cracking of the disc in the region of the aperture, whereupon the disc must be rejected as scrap.
Some attempts have been proposed to mold an aperture in the information disc prior to solidification of the disc-forming material and thereby avoid use of a punch assembly. See, for example, copending and commonly assigned U.S. application Ser. No. 202,824, filed in the name of A. Cane et al., and entitled "HOT SPRUE VALVE ASSEMBLY FOR AN INJECTION MOLDING MACHINE" which discloses a valve assembly positioned across the mold cavity from the sprue bushing and operable to mold the central aperture while controlling flow of molten material into the mold cavity. However, this type of aperture-molding valve assembly requires relatively complex valve actuation devices and requires solidification of at least a portion of the disc-forming material within the region of the aperture and within the sprue bushing before the parting line can be opened and the molded disc removed. Accordingly, minimum disc cycle time is still not achieved and solidified material discarded as scrap is not avoided.
Other attempts have been made to mold an aperture in the information disc without requiring solidification and waste of any portion of the disc-forming material in the vicinity of the formed aperture or within the sprue bushing. See, for example, copending and commonly assigned U.S. Pat. No. 4,340,353 filed in the name of B. F. P. Mayer et al., and entitled "HOT SPRUE VALVE ASSEMBLY FOR AN INJECTION MOLDING MACHINE" which discloses a poppet valve structure movable in response to relatively complex valve actuation devices positioned across the mold cavity from the sprue bushing. However, this movable poppet valve structure has been found to cool relatively quickly during the cooling and solidification of the molten material within the mold cavity, resulting in undesired solidification of portions of the material within the sprue bushing. When this occurs, the solidified portions tend to clog the sprue bushing and prevent proper injection of molten material in the formation of subsequent information discs.
The present invention overcomes the problems and disadvantages of the prior art by providing an injection molding machine including an improved hot sprue valve assembly having simplified valve structure for controlling flow of molten material into the mold cavity and for molding a central aperture into an information disc prior to solidification of the material within a mold cavity. Moreover, the improved hot sprue valve assembly is designed to isolate the molten material within the sprue bushing from the material within the mold cavity to prevent solidification of material within the sprue bushing.